1. Field of the Invention
The present invention relates to a linear direct current motor commonly used for moving an object to be moved with high accuracy in, for example, a motion mechanism such as a machine tool or industrial robot, and more particularly, to a brushless type of linear direct current motor.
2. Description of the Prior Art
FIG. 1 shows a drive unit containing a linear direct current motor of the prior art. Furthermore, this drive unit has a guide unit for guiding an object added to a linear direct current motor.
As shown in the drawing, this drive unit has a long base member 1 and moving body 2 that moves along said base member 1. More specifically, a plurality of rollers (not shown) are provided on moving body 2, and these rollers roll over a track (not shown) formed along the lengthwise direction in base member 1.
On the other hand, the linear direct current motor that composes a drive unit together with the above-mentioned guide unit is composed in the manner described below.
Said linear direct current motor is composed of a primary side, equipped with a large number of armature coils 5 arranged in a row in the lengthwise direction of base member 1 on said base member 1, and a secondary side, having a field magnet 6 (see FIG. 2) attached to the bottom surface of moving body 2 so as to oppose each of said armature coils 5. As shown in the drawing, said field magnet 6 is magnetized so that a plurality, in this case 4, of N and S magnetic poles are alternately arranged in a row along direction P in which moving body 2 is to move, namely the lengthwise direction of base member 1. Furthermore, as shown in FIG. 2, if the width of one magnetic pole of field magnet 6 is taken to be Pm in this example, the open angle width of each armature coil 5 is set to the same Pm, and the interval of the armature coils is set to Pm/3.
In the linear direct current motor of the above-mentioned constitution, by supplying a prescribed excitation current to armature coils 5, thrust is generated based on Fleming's right hand rule between the primary and secondary sides. For example, if base member 1, to which the primary side is coupled, is taken to be the stationary side, moving body 2, integrated into a single unit with the secondary side, is moved by this thrust.
However, in the linear direct current motor as described above, it is important to systematically supply an excitation current to each armature coil to maintain as constant a thrust as possible regardless of changes in the position of the primary side with respect to the secondary side. Continuing, the following provides an explanation of the constitution pertaining to this supply of power.
As shown in FIGS. 3 through 5, magnetic pole discrimination elements in the form of Hall effect elements 8a through 8e are respectively arranged in the vicinity of each armature coil 5a through 5e (these five armature coils are mutually distinguished by adding small letters of the alphabet from a through e to reference numeral 5 indicating armature coils in the explanation thus far for the sake of convenience in the explanation). In this example, each of Hall effect elements 8a through 8e is arranged corresponding to conductors 5a.sub.2 through 5e.sub.2 on one side among the conductors (arms) that contribute to thrust possessed on two sides by each armature coil 5a through 5e. These Hall effect elements 8a through 8e emit a signal (in the form a potential difference) corresponding to the lines of magnetic force emitted by each magnetic pole possessed by field magnet 6 when said field magnet 6 approaches. Electrical power is then supplied to the armature coil corresponding to the Hall effect element that emitted said signal based on that signal. Alternatively, this supply of electrical power is interrupted to the armature coil corresponding to a Hall effect element for which said signal has yet to be obtained or is no longer being obtained, thus enabling control to be performed.
Control of the supply of electrical power is performed in the manner described below based on said constitution.
In FIGS. 3 through 5, the letters (a) through (i) indicate that field magnet 6 is located at each of the positions shown in the drawing corresponding to those letters.
To begin with, in the case field magnet 6 is located at position (a) of FIG. 3, since each of magnetic poles 6b and 6c of said field magnet 6 acts on two Hall effect elements 8a through 8b, the two armature coils 5a and 5b that respectively correspond to these Hall effect elements are supplied with electrical power. Furthermore, although operation after this point is similar, the Hall effect elements that act on the respective boundaries of each magnetic pole (6a and 6d) do not operate. In this state, all conductors, which are possessed on two sides each by each of said armature coils 5a and 5b, contribute to thrust, and these conductors consist of conductors 5a.sub.1, 5a.sub.2, 5b.sub.1 and 5b.sub.2. These are indicated with a circle in FIG. 3. Thrust is generated since these four conductors act on magnetic poles 6a, 6b and 6c of field magnet 6. Furthermore, since conductors 5c.sub.1 and 5c.sub.2 of the other armature coil 5c act on the boundary between corresponding magnetic poles of field magnet 6 (conductors 5c.sub.1 and 5c.sub.2), thrust is not generated even though power is supplied to armature coil 5c.
Next, as shown in FIGS. 3 through 5, in the case the open angle width of the armature coils is taken to be Pm, the interval between the armature coils is taken to be Pm/3, and this is divided into 8 divisions, when field magnet 6 is at position (b) in FIG. 3, namely when moved by 1/8, since each of magnetic poles 6a through 6d of said field magnet 6 acts on three Hall effect elements 8a through 8c, three armature coils 5a through 5c, which respectively correspond to these Hall effect elements, are supplied with electrical power. In this state, conductors possessed on two sides each by each of said armature coils 5a, 5b and 5c all contribute to thrust, and consist of the six conductors 5a.sub.1, 5a.sub.2, 5b.sub.1, 5b.sub.2, 5c.sub.1 and 5c.sub.2. These are indicated with a circle in FIG. 3. Thrust is generated since these six conductors act on magnetic poles 6a, 6b, 6c and 6d of field magnet 6.
Thus, when field magnet 6 is at each of the positions of (c) through (i) shown in FIGS. 3 through 5, electrical power is continued to be supplied to the prescribed armature coils in the same manner as described above. FIG. 16 (a) illustrates the relationship with coil drive current based on the number of coils and the number of arms obtained in this manner.
Although excitation current is supplied to each armature coil in the manner described above in the above-mentioned example of a linear direct current motor of the prior art, the prior art has the problems described below.
Namely, when field magnet 6 is located at position (a) shown in FIG. 3, thrust is actually generated by four of the conductors that contribute to thrust possessed by each armature coil as previously described. However, the number of conductors that generate thrust when field magnet 6 is moved to the other positions of (b) through (i) changes, namely being 6, 4, 5, 3, 5, 3, 5 and 4 conductors, respectively. Thus, a constant level of thrust cannot be obtained at all times due to the wide range of variation. This is clear from FIG. 16 (a).